What Is a Blood Test?

A blood test is when a sample of blood is taken from the body to be tested in a lab. Doctors order blood tests to check things such as the levels of glucose, hemoglobin, or white blood cells. This can help them detect problems like a disease or medical condition. Sometimes, blood tests can help them see how well an organ (such as the liver or kidneys) is working.

What Is a Prothrombin Time Test?

A prothrombin time (PT) test measures how long it takes for a clot to form in a blood sample. Clotting is important to help prevent too much bleeding. Certain proteins (called clotting factors) are essential for blood to clot properly. If there aren't enough of these clotting factors, or any of them don't work as they should, it can take longer than normal for blood to clot.

Why Are PT Tests Done?

PT tests are done to check for bleeding problems. A child might have the test he or she has had a lot of bleeding or bruising, has a medical condition that can lead to problems with clotting, or is having surgery or a procedure that might cause bleeding. PT tests are also done to follow clotting levels in children who are on blood-thinning medicines.

How Should We Prepare for a PT Test?

Your child should be able to eat and drink normally unless also getting other tests that require fasting beforehand. Tell your doctor about any medicines your child takes because some drugs might affect the test results.

Wearing a T-shirt or short-sleeved shirt for the test can make things easier for your child, and you also can bring along a toy or book as a distraction.

How Is a PT Test Done?

Most blood tests take a small amount of blood from a vein. To do that, a health professional will:

clean the skin

put an elastic band (tourniquet) above the area to get the veins to swell with blood

insert a needle into a vein (usually in the arm inside of the elbow or on the back of the hand)

pull the blood sample into a vial or syringe

take off the elastic band and remove the needle from the vein

In babies, blood draws are sometimes done as a "heel stick collection." After cleaning the area, the health professional will prick your baby's heel with a tiny needle (or lancet) to collect a small sample of blood.

Collecting a sample of blood is only temporarily uncomfortable and can feel like a quick pinprick.

Can I Stay With My Child During a PT Test?

Parents usually can stay with their child during a blood test. Encourage your child to relax and stay still because tensing muscles can make it harder to draw blood. Your child might want to look away when the needle is inserted and the blood is collected. Help your child to relax by taking slow deep breaths or singing a favorite song.

How Long Does a PT Test Take?

Most blood tests take just a few minutes. Occasionally, it can be hard to find a vein so the health professional may need to try more than once.

What Happens After a PT Test?

The health professional will remove the elastic band and the needle and cover the area with cotton or a bandage to stop the bleeding. Afterward, there may be some mild bruising, which should go away in a few days.

When Are PT Test Results Ready?

Blood samples are processed by a machine, and it may take a few hours to a day for the results to be available. If the test results show signs of a problem, the doctor might order other tests to figure out what the problem is and how to treat it.

Are There Any Risks From PT Tests?

A PT test is a safe procedure with minimal risks. Some kids might feel faint or lightheaded from the test. A few kids and teens have a strong fear of needles. If your child is anxious, talk with the doctor before the test about ways to make the procedure easier.

A small bruise or mild soreness around the blood test site is common and can last for a few days. Get medical care for your child if the discomfort gets worse or lasts longer.

If you have questions about the PT test, speak with your doctor or the health professional doing the blood draw.